haskell Non-exhaustive patterns in function code

Question

i'm a beginner in haskell… i'm trying to write a function short:: String -> String that encodes a string with only contains the following characters:

moni :: Char -> String
moni 'm' = "01"
moni 'o' = "1"
moni 'n' = "001"
moni 'i' = "11"
short :: String -> String
short x = moni 'x'        -- here i try that short m = "01" but it doesn´t work…. why?)
in ghci: 
short m        -- error variable not in scope 
short "m"      -- error Non-exhaustive patterns in function code

In the end the function short should return only a string of bits … example "omi" should yield "10111"....

Answer 1

If you write 'x', you do not use the variable x, you used a character literal.

If you want to "unpack" a string, such that you retrieve the only character, then you can use:

short :: String -> String
short [x] = moni x

since a String is just a list of Chars. But the above will not work for a string with no characters, or two or more. If you want to map every character to its moni equivalent, and concatenate the result, you can use concatMap:

short :: String -> String
short = concatMap moni